In the fields of microfabrication represented by fabrication of integrated circuit devices, the processing size has become more and more minute in order to achieve higher integration in a lithographic process. In recent years, development of a lithographic process enabling stable microfabrication with a line width of 0.5 μm or less has been actively proceeding.
However, it is difficult to form such a fine pattern with high accuracy using a conventional method utilizing visible rays (wavelength: 800 to 400 nm) or near ultraviolet rays (wavelength: 400 to 300 nm). To deal with this problem, a lithographic process using radiation with a shorter wavelength (wavelength: 300 nm or less) that can achieve a wider depth of focus and is effective for ensuring design rules with minimum dimensions has been proposed. As a lithographic process using radiation with such a short wavelength, processes using deep ultraviolet rays such as KrF excimer laser (wavelength: 248 nm), or an ArF excimer laser (wavelength: 193 nm), X-rays such as synchrotron radiation, and charged particle rays such as electron beams have been proposed. A chemically-amplified resist has attracted attention as a resist exhibiting high resolution for such short wavelength radiation. At present, development and improvement of the chemically-amplified resist are important technical issues of the lithographic process.
The chemically-amplified resist contains a compound which generates an acid upon irradiation. Chemical changes in the resist film (changes in polarity, breakage of a chemical bond, cross-linking reaction, etc.) caused by the catalytic action of an acid change solubility of the exposed area in a developer. A pattern is formed utilizing this phenomenon.
A number of compositions for chemically-amplified resists, for example, a combination of a resin containing an alkali-soluble resin, in which the groups exhibiting affinity with an alkali are protected by a t-butyl ester group or t-butoxy carbonyl group, with an acid generator (see, for example, Patent Document 1); a combination of a resin containing an alkali-soluble resin, in which the groups exhibiting affinity with an alkali are protected by a silyl group, with an acid generator (see, for example, Patent Document 2); a combination of an acetal group-containing resin with an acid generator (see, for example, Patent Document 3); a combination of an alkali-soluble resin, a dissolution controller, and an acid generator, a combination of a novolac resin, a crosslinking agent, and an acid generator; and the like have been proposed.
Patent Document 1: Japanese Laid-Open Patent Publication No. 59-45439
Patent Document 2: Japanese Laid-Open Patent Publication No. 60-52845
Patent Document 3: Japanese Laid-Open Patent Publication No. 2-25850
These chemically-amplified resists, however, have a problem with respect to the process stability, because the compositions are easily affected by moisture, oxygen, basic substances, and the like that are present in the atmosphere for the lithographic process. Non-Patent Document 1, for example, reports that a very small amount of dimethylaniline in the atmosphere deactivates acids existing near the surface among the acids produced in the resist film by exposure, and forms a scarcely soluble layer on the surface of a resist film, which layer remains in the form of an eave-like projection on the surface of a resist pattern after development. Not only such a scarcely soluble layer decreases sensitivity and resolution of the resist, but also the eave-like projections formed in the resist pattern adversely affect the etching accuracy. The size of the eave-like projections tends to increase along with an increase of the time for which the resist is allowed to stand between each of the steps of exposure, post exposure baking, and development. This phenomenon is known as post exposure time delay (hereinafter referred to as “PED”, which unduly decreases the allowance of time in the lithographic process.
Non-Patent Document 1: SPIE, Vol. 1466, “Advance in Resist Technology and Processing,” p. 2 (1991)
As a method for solving such problems of PED, a method of laminating an overcoat on the chemically-amplified resist film to block the film surface from the atmosphere has been proposed. For example, Non-Patent Document 2 describes a method of laminating an overcoat of polyacrylic acid, polyvinyl butyral, polyvinyl alcohol, polystyrene sulfonic acid, or the like on the chemically-amplified resist film to inhibit invasion of basic materials into the resist film, thereby preventing a decrease of sensitivity and resolution of the resist. However, among such overcoats, polyacrylic acid, polyvinyl butyral, and polyvinyl alcohol cannot necessarily effectively prevent formation of the aforementioned scarcely-soluble layer, although they have a barrier effect. Polystyrene sulfonate, which has too strong acidity, has a drawback of initiating a chemical reaction due to the catalytic action of the acid in the chemically-amplified resist irrespective of exposure to light. Another problem with the overcoat, which is commonly applied to a resist film as an aqueous solution, is uneven coating due to insufficient wetting properties of the aqueous solution with the resist film.
Non-Patent Document 2: WO92/05474
On the other hand, since radiation commonly used in the lithographic process is light with a single wavelength, incidence radiation interferes with light reflected by the boundary surfaces on the top and bottom of the resist film. As a result, a phenomenon called “a standing-wave effect” or “a multiplex interference effect”, which is a phenomenon in which an effectual dose of radiation to which a resist film is exposed fluctuates due to mutual interference of radiations in the film according to fluctuation of the resist film thickness, irrespective of a constant actual dose of radiation, occurs and adversely affects formation of resist patterns. If the coating thickness varies due to slight difference of a resist composition, viscosity, resist film conditions, and the like, or the coating thickness fluctuates due to steps in the substrates (recessed parts are thicker than projected parts), such a film thickness difference changes the effectual dose of radiation to which a resist film is exposed, resulting in fluctuation of pattern dimensions and a decrease in accuracy of the resist pattern dimensions.
To overcome this problem caused by the standing-wave effect, a method of forming an antireflection film on the resist film to inhibit reflection on the coating surface and reduce multiplex interference in the film has been proposed. For example, a method of reducing the standing-wave effect by laminating layers of polysiloxane, poly(ethyl vinyl ether), polyvinyl alcohol, or the like as an antireflection film on a resist film is described in Non-Patent Document 3. In this instance, the reflex inhibition effect on the surface of the resist film mainly depends on the refractive index and film thickness of the antireflection film. The refractive index of an ideal antireflection film is the square root on n (n is the refractive index of the resist), and the thickness of an ideal antireflection film is an anisoploid of λ/4m (wherein λ is the wavelength of radiation and m is the refractive index of the antireflection film).
Non-Patent Document 3: J. Eectrochem. Soc., Vol. 137, No. 12, p. 3900 (1990)
However, the antireflection film made from polysiloxane, poly(ethyl vinyl ether), or polyvinyl alcohol has a basic problem of being incapable of sufficiently inhibiting the standing-wave effect due to the small difference of the refractive index with that of a resist film. In addition, the antireflection film made from polysiloxane which is insoluble in water or a developer must have a separate step of removing the antireflection film before development using an antireflection film removing agent. The solubility in water or a developer of poly(ethyl vinyl ether) or polyvinyl alcohol is not necessarily sufficient. The antireflection film made from these polymers may have leave residues on the resist film which impair resist performance such as resolution, developability, pattern profile, and the like. An additional problem of the overcoat which is applied to a resist film as an aqueous solution such as poly(ethyl vinyl ether) or polyvinyl alcohol is uneven coating due to insufficient wetting properties of the aqueous solution with the resist film.
In order to solve the problems in conventional antireflection films mentioned above, the inventor of the present invention has proposed (a) a copolymer of an acrylamide compound such as 2-acrylamide-2-methylpropane sulfonic acid in which the amide group has a sulfonic acid group bonded to the nitrogen atom via an organic group, and a fluoroalkyl acrylate compound such as 2,2,3,3,3-pentafluoropropyl acrylate and (b) a basic material blocking antireflection film containing a fluoroalkylsulfonic acid having an fluoroalkyl group with 5 to 15 carbon atoms and/or a fluoroalkylcarboxylic acid having an fluoroalkyl group with 5 to 15 carbon atoms, and a resist pattern forming method using the antireflection film (see Patent Document 4). This antireflection film can reduce the effect of a basic substance  in the atmosphere and the standing-wave effect. However, the composition used for forming the antireflection film has a problem in coatability onto a resist film, which results in production of microbubbles in the order of μm in the formed coating.
Patent Document 4: Japanese Laid-Open Patent Publication No. 7-234514